The present invention relates to a process for preparing deodorized 1,2-propanediol by distillative workup of the hydrogenation output obtained in the hydrogenation of glycerol.
Processes for preparing raw materials and especially fuels from biogenic fatty or oily starting mixtures and, for example, used oils and animal fats which occur in restaurants have been known for a long time, preference being given to using rapeseed oil as the starting material in the production of biogenic fuels. Biogenic oils and fats themselves are not very suitable as a motor fuel, since they have to be purified beforehand by usually complex processes. These include the removal of lecithins, carbohydrates and proteins, the removal of so-called oil sludge and the removal of the free fatty acids which are present, for example, in relatively large amounts in rapeseed oil. Vegetable oils processed in this way nevertheless deviate from the technical properties of conventional diesel fuels in several aspects. For instance, they generally have a higher density than diesel fuel, the cetane number of rapeseed oil is lower than that of diesel fuel, and the viscosity is several times higher than that of diesel fuels. One known way of solving the associated problems is to convert the triglycerides present in the biogenic oil and fat starting mixtures to fatty acid monoalkyl esters, especially methyl or ethyl esters. These esters, also referred to as “biodiesel”, can generally be used in diesel engines without major modifications. The transesterification of the triglycerides for biodiesel production also produces glycerol, from which 1,2-propanediol can be obtained in a manner known per se by hydrogenation.
For instance, DE-C 524 101 describes such a process, in which glycerol, among other materials, is subjected to a gas phase hydrogenation in the presence of a hydrogenation catalyst with hydrogen in a considerable excess, using bromine-activated copper or cobalt catalysts. DE 4302464 A1 describes a process for preparing 1,2-propanediol by hydrogenating glycerol in the presence of a heterogeneous catalyst, wherein glycerol is passed over a catalyst bed of copper catalyst. PCT/EP2007/0511983 describes a process for preparing 1,2-propanediol, in which a glycerol-containing stream is subjected to a hydrogenation in the presence of a heterogeneous copper catalyst.
The glycerol-containing streams used here may also comprise numerous undesired components, such as sulfuric acid, hydrogen sulfide, thioalcohols, thioethers, carbon oxide sulfide and, for example, nitrogen compounds, e.g. amino acids. Even though the removal of these impurities actually before the hydrogenation of the glycerol is known, some of them are still present in the 1,2-propanediol after the hydrogenation of the glycerol. Such impurities can lead to the 1,2-propanediol obtained being unsuitable for particular applications owing to the odor emanating from them. More particularly, troublesome impurities are thiols, fatty acids, esters, aldehydes and ketones, some of which, being odor-intensive substances, can be perceived even in the ppb range.